Control valves have many different applications, such as controlling pressure downstream from the valve, controlling levels in tanks or reservoirs either upstream or downstream from the valve, reducing undesirable effects of pressure surges in supplies due to actuating related valves or pumps, etc. The valves have a valve element that is modulated to throttle flow through the valve to attain a desired pressure drop or flow condition across the valve. A fluid-filled, variable volume control chamber is sealed with a movable partition means, for example a piston with a dynamic seal or sliding resilient seal, such as e.g., a flat or rolling flexible diaphragm. The dynamic seal isolates the control chamber having low pressure from the main flow chamber usually having higher working pressures. The piston and dynamic seal are movable within the valve body in response to pressure changes within the control chamber. A pilot pressure system is coupled to the control chamber. The pilot system generates at least one type of pressure signal which is processed following conventional techniques and generates a control signal which is fed into the control chamber used to control position of the main valve member. However, the dynamic seal is subject to wear and potential failure due to substantial movement, manufacturing defects, and/or aging.
U.S. Pat. No. 5,348,036 to Oksanen describes an automatic control valve having a main valve member movable between open and closed positions in response to pressure applied to a first piston or diaphragm in a first control chamber to modulate flow through the valve to maintain desired flow conditions. Pilot pressure signals are generated to reflect the flow conditions, usually upstream and downstream from the valve and processed to generate a control signal fed to the first control chamber to control pressure therein. Wear of the valve causes leakage of the diaphragm or seal of the piston causing loss of primary control of the valve member which tends to fail open. An initial rise in flow through the valve is initially caused which is detected by the pilot pressure signal but because of the leakage, is unable to correct. When pilot pressure signals reflect primary control failure, a back-up piston or diaphragm moves to an active position and selectively cooperates with the main valve member to control flow conditions through the valve independently of the first control chamber, without regard to pressure control.